1. Field of the Invention
The invention relates to a machine device having a rotor, which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding, whose conductors are coupled in a thermally conductive manner to a central, cylindrical rotor cavity which extends in the axial direction and is part of a closed pipeline system, in which there is a refrigerant which circulates using a thermosiphon effect, condensed refrigerant passing into the central rotor cavity, and evaporating refrigerant emerging there from the rotor cavity. A corresponding machine device is described in WO 02/43224 A1.
2. Description of the Related Art
Since 1987, metal-oxide superconductor materials having critical temperatures Tc of over 77 K have been known. These materials are therefore also referred to as high Tc superconductor materials or HTS materials and in principle allow a cooling technique using liquid nitrogen (LN2).
Using conductors formed of HTS materials, attempts are also being made to produce superconducting windings for machines. Conductors known to date, however, have a relatively low current-carrying capacity, in particular in magnetic fields having inductions in the Tesla range. It is therefore often necessary, despite the intrinsically high critical temperatures Tc of the materials used, to keep the conductors of such windings at a temperature level, which is significantly below 77 K, of, for example, between 10 and 50 K in order to make it possible to carry significant currents at high field strengths. Such a temperature level is significantly higher than 4.2 K, the boiling point of liquid helium (LHe), at which known metallic superconductor materials having a comparably lower critical temperature Tc, so-called low Tc superconductor materials or LTS materials, are cooled. One suitable refrigerant is therefore, for example, liquid neon (LNe).
For the purpose of cooling windings with HTS conductors in the mentioned temperature range below 77 K, refrigeration equipment in the form of so-called cryogenic coolers having a closed cycle of He compressed gas is preferably used. Such cryogenic coolers are, in particular, of the Gifford-McMahon or Stirling type or are in the form of so-called pulse tube coolers. They make it possible for the cooling power to be made available almost at the press of a button and do not require the use of cryogenic liquids. When using such refrigeration equipment, the superconducting winding is cooled indirectly only by transferring the heat to a cold head of a refrigerator (cf. for example Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16), Kitakyushu, J P, 20–24, May 1996, Verlag Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for the rotor of an electrical machine as is described in the WO-A specification mentioned initially. The rotor contains a winding having HTS conductors which are located in a thermally conductive winding mount. This winding mount is equipped with a central, at least largely cylindrical, rotor cavity which extends in the axial direction and to the sides of which are connected tubular pipeline parts passing out of the winding mount. These pipeline parts run in a geodetically higher condenser space of a refrigeration unit and form, together with this condenser space and the central rotor cavity, a closed single-pipe pipeline system. Located in this pipeline system is a refrigerant which circulates on the basis of a so-called thermosiphon effect. In this case, refrigerant condensed in the condenser space is passed via the tubular pipeline parts to the central rotor cavity, where it absorbs and vaporizes heat owing to the thermal coupling to the winding mount and thus to the HTS winding. The vaporized refrigerant then passes back via the same pipeline parts to the condenser space where it is recondensed. The cooling power required for this purpose is produced by a refrigeration machine, whose cold head is thermally coupled to the condenser space. The return flow of refrigerant is in this case driven toward the parts of the refrigeration machine which act as the condenser owing to a slight excess pressure in the central rotor cavity acting as the vaporizer part. This excess pressure produced by gas being produced in the vaporizer part and the condensation in the condenser space thus results in the desired refrigerant return flow. Corresponding refrigerant flows are also known in principle from so-called “heat pipes”.
In the case of the known machine using thermosiphon cooling by a corresponding refrigeration unit, the refrigerant is thus transported merely using the force of gravity, so that no further pumping systems are required. If it is desired to provide such a machine device, including a machine and an associated refrigeration unit, on ships or offshore installations, it is often necessary to take into account static skews, a so-called “trim”, of an order of magnitude of up to ±5° and/or dynamic skews of up to ±7.5° in the longitudinal direction. In order to obtain approval for use in a ship, the refrigeration system of such a machine unit on board a marine vessel must thus ensure reliable cooling even under these conditions. If it is desired to permit skews of the machine, there is in this case the risk, however, that a region of the tubular pipeline parts between the central rotor cavity and the refrigeration unit will lie at a geodetically lower level than the central rotor cavity. The result of this is that the refrigerant cannot reach the rotor cavity to be cooled owing to the influence of the force of gravity. Cooling of the machine and thus its operation would thus no longer be ensured.
There are a number of known suggestions for counteracting this risk:                One simple solution is to arrange the machine such that it is inclined with respect to the horizontal, with the result that even in the case of a very large assumed trim or oscillation amplitude there is still a gradient in the direction of the rotor cavity in the thermosiphon pipeline system. A correspondingly inclined arrangement is undesirable in shipbuilding, in particular in the case of larger machine lengths, owing to the large amount of space that is required in this case.        Instead of a single-pipe pipeline system, in which the liquid and the gaseous refrigerants flow through identical pipe parts, two-pipe pipeline systems for refrigerant circulation using a thermosiphon effect are also known (cf. for example WO 00/13296 A1). In this case, however, an additional pipe must be provided for the gaseous refrigerant in the region of the hollow shaft of the rotor. This also requires additional sealing complexity.        In principle, the refrigerant may also be forced to circulate by a pumping system. For this purpose, however, correspondingly high complexity is required in terms of the apparatus, in particular if the refrigerant is intended to be, for example, at a low temperature of, for example, approximately 30 K. Such circulation systems also bring about considerable losses and can scarcely fulfill the service life requirements for shipbuilding with its long maintenance intervals.        
In addition, there is the problem that, when the machine is inclined in the longitudinal direction such that the entry end of the refrigerant into the central rotor cavity is at a geodetically lower level than the axially opposite end, the refrigerant flows in the direction of the region of the refrigerant entry end in response to the force of gravity. It would thus no longer be possible to reliably ensure cooling of the machine and thus its standby operation, since, in this region, thermal losses which are too high and undesirable cooling of the rotor shaft result. In normal operation, i.e. in the normal operating state, this problem does not occur, since in this case, owing to the rotation and possibly the slightly conical design of the central rotor cavity, the refrigerant is forced toward the outer walls and is pushed in the direction of the axially opposite end to the entry end of the refrigerant.